The summary in #11 and Shy Torque's Chetwynd 'event' are,essentially pointing at the same sequence - the direction of divergence being opposed, down to the preferred 'National' rotor rotation direction.
Shy - your QHI was my guide and mentor on my first Whirlwind ride ... which included a Chetwynd visit for sloping ground ... but no RBS demo ! His accident was a huge shock.
I only experienced jack stall as a demo on my first trip at Boscombe (in a Gazelle) and (from memory) it was something of a non-event.